Current studies are focused on the synthesis, modification and function of messenger RNA (mRNA). A novel "cap" structure containing methylated nucleotides has been found at the 5'-end of a variety of viral and cellular mRNAs. This structure is important for ribosome binding and translation of mRNA. Three viral enzyme, a guanylyltransferase, a guanine-7-methyltransferase, and nucleoside-2'-methyltransferase, are needed to form the cap and have been isolated from vaccinia virus cores. Similar activities, as well as an adenine-N6-methyltransferase, that introduces an additional methyl group into the cap have been isolated from uninfected HeLa cells. The steps involved in the post-transcriptional modification of mRNA have been determined from studies of these enzymes. The sequence complexity of vaccinia virus mRNA made at different times after infection has been determined by nucleic acid hybridization in order to understand the regulation of gene expression. Early mRNA is transcribed from only 50% of the genome whereas late mRNA is made from 80%. Early vaccinia virus proteins have been synthesized in vitro using a coupled transcription-translation system. BIBLIOGRAPHIC REFERENCES: Boone, R. F., Ensinger, M. J. and Moss, B: Synthesis of mRNA quanylytransferase and mRNA methyltransferases in cells infected with vaccinia virus. J. Virol. 21: 475-483, 1977. Moss, B., Gershowitz, A., Weber, L. A. and Baglioni, C.: Histone mRNAs contain blocked and methylated 5' terminal sequences but lack methylated nucleosides at internal positions. Cell 10: 113-120, 1977.